Coaxial transmission lines or waveguides are widely recognized as efficient media for transferring electrical signals. Their desirable properties include broad bandwidth and relatively low power loss. Such properties result, at least in part, from the fundamental structure of these media. Coaxial transmission lines or waveguides include an inner or center conductor and outer conductor, sometimes referred to as a shield or wall. Electrical signals can be driven between the inner and outer conductors. Beneficially, the coaxial structure supports the transfer of low frequencies lending towards its broad bandwidth properties. A region between the conductors can be filed by one or more dielectrics, such as air or a vacuum. Electromagnetic radiation is generally confined to this region inside the waveguide, sometimes referred to as “shield effect.” Thus, the transmission of energy in the waveguide occurs through the dielectric inside the waveguide, between the inner and outer conducting surfaces. In radio-frequency applications, for example up to a few gigahertz, the wave propagates primarily in the transverse electromagnetic (TEM) mode, with the electric and magnetic fields both substantially perpendicular to the direction of propagation, which is generally along a central axis. Above a certain cutoff frequency, however, transverse electric (TE) and/or transverse magnetic (TM) higher order modes can also propagate, as they do in a hollow waveguide.
Most of the shield effect in such coaxial waveguides results from opposing currents between an outer surface of the center conductor and an inner surface of the opposing outer conductor, or shield, creating opposite magnetic fields that cancel, and thus do not radiate. Additionally, for circular coaxial transmission line, the electric field is radially symmetric about the center conductor. Electric field lines diametrically opposed from each other would thus be 180 degrees out of phase with respect to each other. Consequently, for an open-ended coaxial waveguide or cable, any radial portion of the electric field exposed to the open end would cancel with its opposing radial portion of the electric field, thus precluding the possibility of far-field radiation. It is just such features effectively preventing radiation from coaxial waveguide structures that contribute to their effectiveness as energy transfer media.
Radiating or “leaky cable” is another form of coaxial waveguide that is constructed with tuned slots cut into the outer shield. These slots are tuned to the specific radio frequency (RF) wavelength of operation or tuned to a specific radio frequency band. This type of cable is used to provide a tuned bi-directional “desired” leakage effect between transmitter and receiver. It is often used in elevator shafts, underground, transportation tunnels and in other areas where other forms of antennas are not feasible. The direction of radiation is broadside to a central axis of the coaxial waveguide and can vary depending on such features of spacing.